In recent years, electrophotographic image forming apparatuses have been used not only as usual copiers or printers for printing or copying intra-office documents but also in the field of preparation of printed materials for extra-office use, specifically, their use has been expanded to the printing on-demand (POD) market in the area of short-run printing. In the POD market, the image quality is required as high as the graphic arts printing for the formed printing material, since required numbers of printed material having high quality image such as photographic image are printed on demand without print making.
In order to obtain printed material of high image quality required in the POD market, reduction of the toner particle diameter is known to be effective and there have been proposed various chemical toners to achieve this. Such a chemical toner, which is prepared by the process of granulation in an aqueous medium, has an advantage that fine toner particles of high uniformity can be obtained, in contrast to a pulverization method.
As is also known, the use of polyester resin as a binding resin for toner particles is effective to obtain printed material of high image quality with high glossiness, without causing an offset phenomenon in fixing.
There was proposed a method of preparing fine toner particles by using a polyester resin, in which the polyester resin dissolved or dispersed in a solvent, was dispersed in an aqueous medium to form oil-droplets, followed by removal of the solvent to obtain toner particles.
The polyester resin can be synthesized via polycondensation, and catalyser is used in the polycondensation in majority. There are generally used tin compounds such as dibutyl tin as a catalyst used for synthesis of polyester resin through polycondensation, and technologies to synthesizing polyester resin employing the tin compound as a catalyser has been examined. (See, for example, Patent Document 1)
However the method of manufacturing toner particles employing the polyester resin synthesized with tin compound as the catalyser includes one wherein toner particles are manufactured by granulating oil droplets which is formed by dissolving or dispersing the colored particles as well as polyester resin in a solvent to form. The dispersibility of the colored particles in the toner particles obtained by this method has a tendency to deteriorate, and therefore, the toner is disadvantageous to make printed matter having broad color reproduction range.
Further, the tin compounds used as a catalyst are organo-tin compounds having an aliphatic group bonded to metal (tin). Recently, organo-tin compounds may be subjected to regulation to use in view of environmental suitability. The catalyst used to synthesize of polyester resin has been reexamined.
Recently, in view of such environmental consideration, there were proposed titanium catalysts such as titanium halogenate, titanium diketoenolato, titanium carboxylate, titanyl carboxylate and titanyl carboxylate salt; and metal catalysts such as a germanium catalyst and an aluminum catalyst, as disclosed in Patent Documents No. 2 to 4.
The toner employing the polyester synthesized employing the catalysts mentioned above observed tendency to improve charging rise up by virtue of an action of the metal element. On the other side, it becomes difficult to maintain the stability of charging performance due to an effect by metal atom, and it becomes difficult to maintain the charging performance particularly in case that the image is formed under the condition of high temperature and high humidity.
As it becomes difficult to maintain the stability of charging performance due to an effect by metal atom contained in the polyester resin, it becomes difficult to supply a predetermined amount of toner on the surface of the photoreceptor, and consequently, image density or color reproduction is affected. Particularly it concerns about use in for on demand printing which strictly requires density and color hue.